Sydney has been deluged by the heaviest November rain it has experienced in decades, causing flash-flooding, traffic chaos and power cuts.

Heavy rain fell throughout Wednesday, the city at one point receiving its average monthly rainfall in two hours.

Officials said dozens of vehicles crashed in the stormy conditions, with winds of up to 90km/h (55mph) also recorded.

At least two deaths have been blamed on the storm.

A 14-year-old boy died in a car accident in Thornleigh, in the north of Sydney on Wednesday morning.

A volunteer with the state's emergency service also died while out on duty, and two police officers were injured after being hit by a falling tree while trying to help Sydney motorists, authorities said.